Inverted telephoto type wide angle lens system

ABSTRACT

An inverted telephoto type wide angle lens system comprises, in order from the object side, a negative meniscus lens having its convex surface facing the object side, a first positive lens, a negative lens, a second positive lens and a third positive lens. The lens system satisfies certain conditions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in inverted telephoto type wideangle lens systems.

2. Description of the Prior Art

Generally, inverted telephoto lenses are advantageous as wide anglelenses for single lens reflex cameras, but distortion becomes morepronounced as the short focal length lens has a greater angle of view.To correct this, it has usually been the practice to incorporate in thedivergent group a member having a positive refractive power, but thishas increased the size of the entire lens system and increased thenumber of lens components as well as the weight of the lens system. Thisis inconvenient to the portability of the lens system and verydisadvantageous in production.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an invertedtelephoto type wide angle lens system of simple construction which isvery small in size and yet has a relative aperture of 1:2.8, an angle ofview of 74° and a back focus 1.29 times or greater than the focal lengthof the lens system.

The above object and other features of the present invention will becomemore fully apparent from the following description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the lens system according to thepresent invention.

FIGS. 2A-2C graphically illustrate the aberrations in a first embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, the inverted telephoto type wide angle lens systemaccording to the present invention comprises five components which are,in order from the object side, a negative meniscus lens L1 having itsconvex surface facing the object side, a biconvex positive lens L2, anegative lens L3, and two positive lenses L4 and L5 which are alsomeniscus lenses. The lens system satisfies the following conditions:

    0.25f<d.sub.4 <0.5f                                        (1)

    1.6d.sub.4 <d.sub.2 <2.5d.sub.4                            (2)

    d.sub.3 <0.5d.sub.4                                        (3)

where d₄ is the air space between the positive lens L2 and the negativelens L3, d₂ is the air space between the negative meniscus lens L1 andthe positive lens L2, d₃ is the center thickness of the positive lensL₂, and f is the total focal length of the entire system.

Forming a divergent group by a simple negative meniscus lens as in thepresent invention is highly effective to reduce the size and weight ofthe lens system and to reduce the number of the lenses used. On theother hand, because it is a short focal length lens, such lens systemcreates pronounced negative distortion as the result of its strongdiverging action. To correct this, a member having a positive refractivepower has been added as part of the divergent group or the divergentgroup has been formed by two negative lenses, as is well-known in theart. According to the present invention, free from such conventionaltechniques, there has been found the possibility of providing asufficiently good aberration balance by a novel technical means. Morespecifically, in the lens construction of the present invention, theonly convergent air space d₄ which affects the distortion is large, asseen in condition (1) above.

Such means is based on the design concept that employs the positive lensL2 as a kind of distortion correcting lens, and it leads to a veryuseful effect in that it eliminates the need to use a combination ofmore than one lens as heretofore known, such as for example, the need toprovide a positive member in the divergent group. However, the largeconvergent air space d₄ results in overcorrection of the astigmatism,but this can be overcome by a means which will hereinafter be described.

Each of the individual conditions will be described in detail. If theupper limit of condition (1) above is exceeded, the distortion may befurther corrected, but a large overcorrection of the astigmatism willresult. On the other hand, if the lower limit of condition (1) isexceeded, negative distortion will be increased and it becomesimpossible to form the divergent group by a single negative lens.Condition (2) is intended to maintain the correction of the distortionwhile securing a predetermined back focus as the lens for single lensreflex camera. If the upper limit of this condition is exceeded, thenegative distortion will be increased due to the property of thedivergent space and at the same time, the size of the entire lens systemwill become bulky, contrary to an object of the present invention. Ifthe lower limit of condition (2) is departed from, the back focus willbe shortened and make the lens system unusable as the lens for a singlelens reflex camera. Condition (3) prevents the aforementionedovercorrection of the astigmatism which may result from condition (1),and if condition (3) is exceeded, no good correction may be obtained.

By the described construction of the present invention, a simpleinverted telephoto type wide angle lens has been realized which has arelative aperture of 1:2.8, an angle of view of 74° and a back focus1.29 times or greater than the total focal length of the entire systemand yet is very small in overall size and comprises only five lenscomponents.

Further, in the conventional wide angle lens of this type, the centerthickness of the positive lens just before the diaphragm, has tended tobe considerably large, and it has been difficult to make the positivelens as a single lens. For example, the positive lens have been formedof two planoconvex lenses, but in the lens system of the presentinvention, the center thickness d₃ of the positive lens L2 can be madequite small as a result of condition (3) described above. This leads togreater ease with which such positive lens is produced, which in turnleads to a reduced size of the entire lens system and a furtherreduction in the cost.

In the present invention, some desirable conditions as follows may beindicated in addition to the above-described characteristic conditions.To provide a sufficient back focus, the following conditions may beadopted: for the focal length f₁ of the negative meniscus lens L1;

    f<|f.sub.1 |<2f                          (4)

and for the focal length f₂ of the positive lens L₂ which contributes tothe correction of the distortion;

    0.8f<f.sub.2 <1.1f                                         (5)

where f is the total focal length of the entire system.

To provide good correction of the sine condition, the followingcondition may be adopted:

    r.sub.3 <|r.sub.4 |

where r₃ is the curvature radius of the object side surface of thepositive lens L2 and r₄ the curvature radius of the image side surfaceof the same lens.

Further, to provide good correction of the sagittal image plane, the airspace d₆ between the negative lens L3 and the positive lens L4 shoulddesirably be made smaller than 1/2 of the center thickness of any of thenegative lens L3 and the two positive lens L4 and L5. To provide muchbetter correction of the distortion, the two positive lenses L4 and L5should desirably be formed with their more curved surfaces facing theimage side.

FIGS. 2A-2C show the aberrations in a first embodiment of the invertedtelephoto type wide angle lens according to the present invention.Various numerical data of two embodiments of the present invention areshown below. In these embodiments, r₁, r₂, . . . , r₁₀ are the curvatureradii of the successive lenses from the object side, d₁, d₂, . . . , d₉are the center thicknesses of and the air spaces between the individuallenses, n₁, n₂, . . . , n₅ and ν₁, ν₂, . . . , ν₅ are the refractiveindices and Abbe numbers, respectively, of the various lenses.

First Embodiment:

Total focal length f=1.0

Relative aperture 1:2.8

Angle of view 74°

    ______________________________________                                        r.sub.1 = 1.266                                                                         d.sub.1 = 0.07                                                                           n.sub.1 = 1.67025                                                                         ν.sub.1 = 57.5                            r.sub.2 = 0.525                                                                         d.sub.2 = 0.576                                                     r.sub.3 = 0.825                                                                         d.sub.3 = 0.094                                                                          n.sub.2 = 1.60323                                                                         ν.sub.2 = 42.5                            r.sub.4 = -1.696                                                                        d.sub.4 = 0.301                                                     r.sub.5 = 0.563                                                                         d.sub.5 = 0.112                                                                          n.sub.3 = 1.7847                                                                          ν.sub.3 = 26.1                            r.sub.6 = 2.273                                                                         d.sub.6 = 0.026                                                     r.sub.7 = -2.098                                                                        d.sub.7 = 0.073                                                                          n.sub.4 = 1.80411                                                                         ν.sub.4 = 46.6                            r.sub.8 = -0.647                                                                        d.sub.8 = 0.004                                                     r.sub.9 = -17.482                                                                       d.sub.9 = 0.087                                                                          n.sub.5 = 1.732                                                                           ν.sub.5 = 51.0                            r.sub.10 = -0.839                                                             ______________________________________                                    

Back focus B.f.=1.295

The numerical data for the second embodiment of the invention (notillustrated) is as follows:

Total focal length f=1.0

Relative aperture 1:2.8

Angle of view 74°

    ______________________________________                                        r.sub.1 = 1.419                                                                         d.sub.1 = 0.057                                                                          n.sub.1 = 1.6228                                                                          ν.sub.1 = 56.9                            r.sub.2 = 0.501                                                                         d.sub.2 = 0.623                                                     r.sub.3 = 0.901                                                                         d.sub.3 = 0.118                                                                          n.sub.2 = 1.62374                                                                         ν.sub.2 = 47.0                            r.sub.4 = -1.459                                                                        d.sub.4 = 0.327                                                     r.sub.5 = -0.588                                                                        d.sub.5 = 0.122                                                                          n.sub.3 = 1.7552                                                                          ν.sub.3 = 27.5                            r.sub.6 = 1.899                                                                         d.sub.6 = 0.021                                                     r.sub.7 = -6.261                                                                        d.sub.7 = 0.087                                                                          n.sub.4 = 1.713                                                                           ν.sub.4 = 53.9                            r.sub.8 = -0.633                                                                        d.sub.8 = 0.004                                                     r.sub.9 = -9.913                                                                        d.sub.9 = 0.073                                                                          n.sub.5 = 1.77279                                                                         ν.sub.5 = 49.4                            r.sub.10 = -1.110                                                             ______________________________________                                    

Back focus B.f.=1.309

It is believed that the advantages and improved results of invertedtelephoto type wide angle lens system of the invention will be apparentfrom the foregoing description of several preferred embodiments thereof.Various modifications may be made without departing from the spirit andscope of the invention as sought to be defined in the following claims.

I claim:
 1. An inverted telephoto type wide angle lens systemcomprising, in order from the object side, a negative meniscus lenshaving its convex surface facing the object side, a first positive lens,a negative lens, a second positive lens and a third positive lens, saidlens system satisfying the following conditions:

    0.25f<d.sub.4 <0.5f

    1.6d.sub.4 <d.sub.2 <2.5d.sub.4

    d.sub.3 <0.5d.sub.4,

where d₂ is the air space between the negative meniscus lens and thefirst positive lens, d₃ is the center thickness of the first positivelens, d₄ is the air space between the first positive lens and thenegative lens, and f is the total focal length of the entire system. 2.A lens system according to claim 1, wherein said first positive lens isa biconvex lens having its more curved surface facing the object side,said lens system further satisfying the following conditions:

    f<|f.sub.1 |<2f

    0.8f<f.sub.2 <1.1f

where f₁ and f₂ are the focal lengths, respectively, of the negativemeniscus lens and the first positive lens.
 3. A lens system according toclaim 2, wherein the air space between the negative lens and said secondpositive lens is smaller than 1/2 of the center thickness of any of saidsecond positive lens and said third positive lens, and said secondpositive lens and said third positive lens are positive meniscus lenseshaving their convex surfaces facing the image side.
 4. A lens systemaccording to claim 3, and further having the following numericaldata:Total focal length f=1.0 Relative aperture 1:2.8 Angle of view 74°Back focus B.f.=1.295

    ______________________________________                                        r.sub.1 = 1.266                                                                         d.sub.1 = 0.07                                                                           n.sub.1 = 1.67025                                                                         ν.sub.1 = 57.5                            r.sub.2 = 0.525                                                                         d.sub.2 = 0.576                                                     r.sub.3 = 0.825                                                                         d.sub.3 = 0.094                                                                          n.sub.2 = 1.60323                                                                         ν.sub.2 = 42.5                            r.sub.4 = -1.696                                                                        d.sub.4 = 0.301                                                     r.sub.5 = -0.563                                                                        d.sub.5 = 0.112                                                                          n.sub.3 = 1.7847                                                                          ν.sub.3 = 26.1                            r.sub.6 = 2.273                                                                         d.sub.6 = 0.026                                                     r.sub.7 = -2.098                                                                        d.sub.7 = 0.073                                                                          n.sub.4 = 1.80411                                                                         ν.sub.4 = 46.6                            r.sub.8 = -0.647                                                                        d.sub.8 = 0.004                                                     r.sub.9 = -17.482                                                                       d.sub.9 = 0.087                                                                          n.sub.5 = 1.732                                                                           ν.sub.5 = 51.0                            r.sub.10 = -0.839                                                             ______________________________________                                    

where r₁, r₂, . . . , r₁₀ are the curvature radii of the successivelenses from the object side, d₁, d₂, . . . , d₉ are the centerthicknesses of and the air spaces between the individual lenses, n₁, n₂,. . . , n₅ and ν₁, ν₂, . . . , ν₅ are the refractive indices and Abbenumbers, respectively, of the various lenses.
 5. A lens system asdefined in claim 3, and further having the following numericaldata:Total focal length f=1.0 Relative aperture 1:2.8 Angle of view 74°Back focus B.f=1.309

    ______________________________________                                        r.sub.1 = 1.419                                                                         d.sub.1 = 0.057                                                                          n.sub.1 = 1.6228                                                                          ν.sub.1 = 56.9                            r.sub.2 = 0.501                                                                         d.sub.2 = 0.623                                                     r.sub.3 = 0.901                                                                         d.sub.3 = 0.118                                                                          n.sub.2 = 1.62374                                                                         ν.sub.2 = 47.0                            r.sub.4 = -1.459                                                                        d.sub.4 = 0.327                                                     r.sub.5 = -0.588                                                                        d.sub.5 = 0.122                                                                          n.sub.3 = 1.7552                                                                          ν.sub.3 = 27.5                            r.sub.6 = 1.899                                                                         d.sub.6 = 0.021                                                     r.sub.7 = -6.261                                                                        d.sub.7 = 0.087                                                                          n.sub.4 = 1.713                                                                           ν.sub.4 = 53.9                            r.sub.8 = -0.633                                                                        d.sub.8 = 0.004                                                     r.sub.9 = -9.913                                                                        d.sub.9 = 0.073                                                                          n.sub.5 = 1.77279                                                                         ν.sub.5 = 49.4                            r.sub.10 = -1.110                                                             ______________________________________                                    

where r₁, r₂, . . . , r₁₀ are the curvature radii of the successivelenses from the object side, d₁, d₂, . . . , d₉ are the centerthicknesses of and the air spaces between the individual lenses, n₁, n₂,. . . , n₅ and ν₁, ν₂, . . . , ν₅ are the refractive indices and Abbenumbers, respectively, of the various lenses.